Mean Switching Frequency Locking in Stochastic Bistable Systems Driven by Periodical Force

The nonlinear response of noisy bistable systems driven by strong amplitude periodical force is investigated by physical experiment. The new phenomenon of locking of the mean switching frequency between states of bistable system is found. It is shown that there is an interval of noise intensities in which the mean switching frequency remains constant and coincides with the frequency of external periodic force. The region on the parameter plane ``noise intensity - amplitude of periodic excitation'' which corresponds to this phenomenon is similar to the synchronization (phase-locking) region (Arnold's tongue) in classical oscillatory systems.